Electronic switching circuit



June 1, 1954 R, D. THOMPSON ELECTRONIC SWITCHING CIRCUIT 2 Sheets-Sheet1 Filed July 25, 1952 INVENTOR. ROGER D. THOMPSON A TTORNE YS Filed July25, 1952 June. 1, 1954 R, D. THOMPSON ELECTRONIC SWITCHING CIRCUIT 2Sheets-Sheet 2 F /'g. 2 INVENTOR. ROGER 0. THOMPSON A TTORNEYS PatentedJune 1, 1954 UNITED STATES PATENT OFFICE 2,680,195 ELECTRONIC SWITCHINGCIRCUIT Roger D. Thompson, Pompton Plains, N. 5., as-

signor to Allen B.

Du Mont Laboratories, 1110.,

Clifton, N. J., a corporation of Delaware Application July 25, 1952,Serial No. 300,913

4 Claims.

This invention relates to electronic switching circuits of the type usedfor field sequential color television systems.

A primary object of the invention is to proby the previous one. posed inthe triggering and integrating, or delay, and third multivibrators to ofoperation.

The three sets of rectangular pulses provided by the multivibrators areapplied to three amcircuits in the second insure proper sequence for aphotoelectric pickup The first multivibrator consists of tubes l I theusual way, with the control grid l3 of tube H coupled to the anode oftube l2 and the control grid M of tube 7 l2 coupled to the anode of tubeH. In addition,

to the three and ground, and the tubes connected in the same way l9 and2! are interas tubes ii and I2,

28 of a tube 29.

Tube 29 and a companion tube 35 form the third multivibrator and arealso interconnected in the same way as tubes H and l2, 1. e., the anodeof tube 3! is connected to the control grid 32 of tube 29 and the anodeof tube 29 is connected to the control grid 33 of tube 3!. A condenser.M is connected in parallel with the plate load 3% of tube 3|, and thecontrol grid 32 is connected to the suppressor grid 31 or" a tube 38.

grid 4| of tube 38 and the tube is connected to the suppressor grid itof tube ll.

Three amplitude control tubes are connected multivibrators to be turnedon and off thereby. The control grid of tube &2 is connected to thecontrol grid M, the control grid of tube 43 is connected to the controlgrid 23, and the control grid of tube M is connected to the control grid33. The cathode of tube 42 is the cathode of tube 44 is grounded througha gain control potentiometer as. The anodes of these three amplitudecontrol tubes are connected in parallel with the photo-cathode 49 of aphoto-multiplier tube 5!.

The output circuit of a source 52 of negatively polarized, verticaldrive signal voltage pulses is the coupling between the anode thecontrol grid 57 of tube 56 and further coupling between the anode 58 oftube 55 and the suppressor grid 59 of tube 5 5. The second section oftube 56 is connected as a diode between the suppressor grid 5!) andground to serve as an amplitude limiting device and a delay adjustingpot is included in the coupling circuit between anode 58 and grid 59.The anode of tube 54 is connected by means of a differentiating circuit52, consisting of a condenser 63 and a resistor 64, to the threemultivibrators by means of the suppressor grid I! in tube I 2.

the suppressor grid 25 in tube 2| and the suppressor grid 35 in the tube31.

Although the circuits described herein may operate with equal utility inother electrical systems, they are particularly adapted to the socalledfield sequential color television system. The signals supplied by source52 recur at the vertical repetition rate and may be verticalsynchronizing signals or vertical drive signals or the like, whereas thesignals supplied by source 39 recur at the vertical repetition rate andare known as color synchronizing signals.

In describing the operation of the circuit it will be assumed that thefirst multivibrator has just been triggered into its active state i. e.,tube 42 is conductive and tube ii is nonconductive. Tube 32 is alsoconductive since it is controlled to operate simultaneously with tube12.

During the active state of the first multivibrator, I

the first positive peak Figure 2) is generated at the plate of tube Hand at the grid l of tube {2. Because of the sequential nature of theoperation it follows that the second and third multivibrator must be intheir quiescent condition in which tubes and 29 are conductive whiletubes 2! and 31 as well as tubes 13 and M are nonconductive.

In addition, tube 54 is conductive, while its companion tube 56 in themultivibrator circuit in which they are associated is non-conductive, asis tube 33. If all of these conditions are not fulfilled at theinstantof putting the circuit into operation, the synchronizing circuits will,as shown hereinafter, insure that they are reached within the space of afew operating cycles of the ring circuit.

Near the end of the active period of the first multivibrator a pulse 6!from the source 52 is applied to the grid 53, halting current throughthe tube 54, which, in turn, initiates conduction in tube 56 bysupplying a positive-going signal to grid 5?. Shortly thereafter, at atime dete'- mined by the potentiometer 6|, tube 54 resumes conduction,cutting ofi the current fiow through the anode 58 in tube 56. Thisoperation, repeated at the occurrence of each of the pulses E1,generates the pulses 68 at the anode of tube 54, as shown. The advantageof producing those new pulses 68 lies in the control which may bemaintained over their duty cycle by means of control 6|. In order toshape the pulses 5B for further use, they are applied to thedifferentiating circuit 62, from which they emerge as a series ofdifferentiated pulses comprising negative pips 59. The differentiatedpulses are applied to the grid l! of tube !2, which since it isconductive is not affected by the positive pips but is biased beyondcutoff by the negative pips 69. The suppressor grids and are connecteddirectly to grid 11 so that pips 89 render non-conductive whichever oneof the corresponding tubes 2! or 3! is carrying current. Under normaloperating conditions, conduction will be taking place in only one of thetubes !2, 2|, or 31, at a time, stoppage of the current flowtherethrough causes the multivibrator in which it is connected to revertto its quiescent condition. At the instant under consideration thereversion of the first multivibrator to its quiescent state terminatesthe positive portions of the pulses B6 and stops conduction through thetube 42.

The pulse 65 is applied to the suppressor grid 18 through a couplingcircuit !5, the, components of which are chosen to have adifferentiating eifect on the pulse 66, thereby producing a wave of thewave as (shown in "ii in which the negative peak 12 has sufficientamplitude to trigger the second multivibrator by biasing the grid itbeyond the cutoff point. Tube TM is switched to the conductive state atthe same time, and the second multivibrator starts to produce thepositive, or active, portion of the wave 13 which renders tube 33conductive.

At the end of a further period of time determined by the repetition rateof the synchronizing signals 68, tube 54 is again renderednon-conductive and a second one of the negative pips 69 generated,stopping conduction in the tube 2i by forming a negative bias on thegrid 25. The completed rectangular wave 13 is transmitted to the grid 28in tube 29 and is differentiated, as was wave 66. The negative portion14 of the differentiated wave halts conduction in tube 29 to trigger thethird multivibrator into operation in the same way as the active statewas initiated in the second multivibrator. Activation of the thirdmultivibrator starts the active portion of the wave "i8 which, in turn,places tube 44 in a conducting state.

At the occurrence of the proper one of the pulses 69 to terminate thepositive portion of pulse it, the grid 35 is biased so as to cut off thecurrent flow in the tube 3! and return the third multivibrator to itsquiescent state. Instead of obtaining the synchronizing signal for thesucceeding multivibrator which, in this case, is the first multiyibratorfrom the plate of tube 29 in a manner similar to and secondmultivibrators, the wave 11 is obtained from the control grid 32 andapplied to the suppressor grid 3! of tube 38. This latter wave is notdifferentiated as were waves 66 and it.

The biasing of tube 38 is so arranged that grid 37 will allow conductionthereof only during the positive portion of the wave ll. During thistime, a pulse "d9 from the source 39 is applied to the control grid llof tube 38 to coact with wave i! to place tube 38 in a conductivecondition. The resultant output wave 84 is applied to the grid 15 oftube i i to stop conduction therethrough and switch the firstmultivibrator into its active state. This completes one cycle of thering circuit.

The advantage of the tube 38 connected in series with the threemultivibrators in the ring circuit becomes apparent when it is realizedthat, even if the originally imposed conditions were not fulfilled, andif the second, instead of the first, multivibrator were initially in itsactive state, no synchronizing signals could feed through to the firstmultivibator until a pulse E9 allowed tube 38 to become conductive, andthe ring counter circuit would simply mark time until pulse 19 occurred.

It will be noted that only a single diode 22 has been provided in thering portion of the circuit. This diode is polarized so as to clip oilthe most positive peaks of the wave H such as occur at the beginning ofthe active time of the first multivibrator. If these positive portionswere not clipped off by the diode, there would be a possi-v biiity oftheir being applied to the grid 18 during the portion when tube to wasconductive and being amplified therein sufficiently to form nega-. tivepulses which could then be applied to the, suppressor grid 28, causingtube 2 to become nonconductive. Hence, tubes H and 29 would be triggeredat the same time and tubes 42 and 4.4 would become conductivesimultaneously. Due, to the momentary conduction of tube 38, there;

that used in the first.

is no possibility of similar feed-through from the second multivibratorto the first or from the third multivibrator to the second. Hence asingle clipping diode 22 is sufficient.

The effect of the condensers 2i and 34 is to serve as delay means toslow down the operation of the second and third multivibrators in orderto insure that no overlap occurs during which two or more of theamplitude control tubes 42-44 would be simultaneously conductive. Theslowing down effect is exhibited as an increase of the rise and delaytimes of pulses l3 and 16 as shown in the wave form drawing in Figure 2.It is unnecessary to provide such a retarded action in the firstmultivibrator due to the synchronizing sequence in which pulses 19 aretimed to occur after the start of the positive portion of wave 17.Consequently, no condenser has been provided across the plate load oftube I2.

Amplitude control tube 42 functions during the active portion '66 as avariable tive portion of wave 15.

The necessity of preventing overlap between the above-mentioned activeportions has been stressed; in addition, it is essential that theeifective load impedance presented by tubes 42-44 be controlled only bythe corresponding bias control resistors 48-48 and not by the wave shapeof the voltage pulses applied to the control grids. For this reason, thecontrol grids of tubes 42-44 are connected directly to the control gridsof the second tubes in the three corresponding multivibrators. Thevoltage on these grids is either zero (during the active periods of themultivibrators) or far below cutoff bias so that tubes 42-44 are eitherconductive to the limit imposed by resistors 46-48, or they arenon-conductive.

The switch circuit described hereinabove is illustrated as part of afield sequential color television mixing circuit in a flying spotscanner, but its utility is not limited to the single application shown.Some type of mixing circuit is usually incorporated in every colortelevision system to correct the unequal overall color response byproviding an unequal system gain for the three signals corresponding tothe three primary colors. In sequential color systems, the three signalsmay be applied to three separate gain-controlled amplifiers and theoutput signal selected sequentially from each of the amplifiers in turnby means of a switch circuit such as has been described hereinabove, orthe gain (or attenuation) of some element in the composite signal pathThe element to be operated on is the photosensitive pick-up device 55,at least one of which is found in every television system. Since thesystem under consideration is a flying spot scanner, device as the93l-A, which is shown illustratively in Figure 1 with only a few of thenormal number of diodes. The paralleled plate circuits of the controltubes 42-44 are connected to the photocathode 49 of tube 5!, and theoverall gain of the pick-up tube is controlled by the instantaneousimpedance of the tubes 42-44. It is essential, as has already beenstated, that only one of the amplitude control tubes 42-44 be conductiveat a time.

Furthermore, due to the inherent capacity,

" of said generators to indicated by the reference character 84,coupling the photocathode 49 to the output anode 86, any sharp voltageimpulses that existed at cathode 49 would be impressed on the videosignal at anode 86. In order to suppress any such impulses, a condenser87 is connected between cathode 49 and ground.

Although this invention has been described in relation to a particularcircuit arrangement, it is not to be considered limited thereby but onlyby the following claims.

What is claimed is:

l. A switching arrangement comprising a first source of synchronizingsignals having a predeequal to an integral multiple of the repetitionrate of said first-named synchronizing signals; and a plurality of pulsegenerating circuits equal in number to said integral multiple, each ofsaid circuits being connected to one other of said cir cuits partiallyto control the operation thereof, said second source being connected toeach of generating circuits, said first source being connected to saidgating circuit to control the operation thereof.

2. A switching arrangement comprising a first source of synchronizingsignals having a predetermined repetition rate; a second source ofsynchronizing signals having a repetition rate equal to three times therepetition rate of said firstnamed synchronizing signals; and threepulse generating circuits, each of said circuits being connected toanother of said circuits partially to control the operation thereof,said second source being connected to each second one of saidgenerators, said first source being connected to said gating circuit tocontrol the operation thereof to allow said second one control the firstone of said generators; and a limiting circuit in the connection betweensaid first generator and a third one of said pulse generating circuits.

3. A switching circuit comprising a first source of synchronizingsignals having a predetermined repetition rate; a second source ofsynochronizing signals having repetition rate equal to an integralmultiple of the repetition rate of said first of said circuits furtherto control the operation thereof, the connection between a first and asecond one of said circuits including a gating circuit; and a connectionbecomprising a smoothing circuit.

4. A switching circuit comprising a first source of synchronizingsignals having a predominating nection between said first one of saidcircuits and a third one of said circuits; a connection between saidthird one of said circuits and said first one of said circuits, saidthird connection including a gating circuit; a connection between saidfirst source and said gating circuit to control the operation thereof; aconnection between said second source and each of said pulse generatingcircuits partially to control the operation thereof; a smoothing circuitcomprising said second connection; an amplitude limiting circuitcomprising said first connection; said third connection comprising asmoothing circuit.

Number Name Date Hochwald May 15, 1951

